1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory.
2. Related Art
A flash memory that is a nonvolatile semiconductor memory is normally a nonvolatile memory that does not need an electric retaining operation (a retention power supply) for storage. Accordingly, programs and the likes can be readily written in a flash memory after the completion of the product, and flash memories are often used in electronic apparatuses. However, there is a demand for a further reduction in size and low-voltage operations in NAND flash memories developed after the next generation.
In each memory cell of a NAND flash memory, a source region and a drain region are formed at a distance from each other on a silicon substrate. A stacked structure is formed on the channel region portion of the silicon substrate located between the source region and the drain region. The stacked structure is formed with a tunnel insulating film, a charge trap film made of silicon nitride, a charge block film made of an insulating material, and a control electrode. If the charge trap film is made of polysilicon, it is also called a floating gate film (FG film). In that case, the charge block film is called an IPD (Inter-Poly Dielectric) film.
In an erasing operation, a high negative voltage is normally applied to the control electrode, so as to release the electrons from the charge trap film (or the FG film) toward the channel region. At this point, electrons might be injected into the charge trap film from the control electrode. To prevent electron injection from the control electrode during an erasing operation, the control electrode should have a large work function.
To apply a high voltage to the tunnel insulating film, the charge block film (or the IPD film) is made of an oxide dielectric material with a high dielectric constant. It is a known fact that, if a metal having a large work function is used as the control electrode, the Fermi level is pinned in the mid-gap direction of Si. Due to this phenomenon, even if the control electrode has a large work function, the effective work function tends to become smaller, and a material or a structure having a sufficiently large effective work function is not obtained.
Meanwhile, in a writing operation, a high positive voltage is normally applied to the control electrode, so as to introduce the electrons into the charge trap film (or the FG film) from the channel region. At this point, holes might be injected from the control electrode. To prevent hole injection from the control electrode during a writing operation, the control electrode should not have a very large work function.
The effective work function becomes smaller than the original work function, because an oxygen defect is caused in a high-dielectric oxide. To solve this problem, an oxide electrode may be used as the control electrode. For example, JP-A 2007-165468 (KOKAI) discloses the use of an oxide electrode. In JP-A 2007-165468 (KOKAI), an oxide conductor such as SrRuO3 is used as an electrode.
In a case where the technique disclosed in JP-A 2007-165468 (KOKAI) is utilized, it is extremely difficult to adjust the effective work function to an optimum value, because 1) the work function of a metal is fixed, and 2) the bandwidth of a metal is large. If the bandwidth is large, the Fermi level hardly varies even when electrons are injected or released. To achieve the high performance expected in a NAND flash memory developed after the next generation, or to perform high-speed writing and high-speed erasing, it is necessary to develop a technique of optimizing the effective work function of the control electrode.